Presented by:
P.Usharani

---

[attachment=14468]
ABSTRACT
The large-scale harvesting of solar energy is an important action to decelerate the observed climate changes. Reliably operating solar-energy systems composing of solar arrays and their interfacing converters are of prime importance to maximize solar-energy harvesting. The paper investigates the solar-generator interfacing in terms of current fed (CF) maximum-power-point (MPP) tracking converter. When the output voltage or current has to be controlled constant, the converter may become unstable at the MPP due to the negative incremental resistance appearing at its input terminals. The operating range of CF converter has to be limited to the voltage less than the MPP voltage, when the output voltage or current control is active.
INTRODUCTION:
The observed climate changes, assumed to be originated from the greenhouse gasses produced by the human activities, have increased the public and political awareness as well as concern on global warming. The awareness on the limited supply of fossil fuels and the global warming problem have created a political climate, which promotes the utilization of renewable and clean energy supplies such as photovoltaic (PV), wind, wave, and fuel-cell energy supplies [1]-[4].
The terminal characteristics of the solar generator imply that the series regulation can be accomplished either by using voltage fed (VF) or current fed (CF) converters because of the constant current type source behavior at the voltage less than the MPP and the constant voltage type source behavior at the voltages higher than the MPP, respectively.
The use of VF MPP-tracking converters is actually quite problematic because the tracking control has to be based on the on the input current of the converter, which has to be always less than the photocurrent of the solar generator for avoiding the saturation of the control. In order to avoid the named difficulties, the input voltage of an MPP-tracking converter is recommended to be controlled instead of the input current. As a consequence, the VF converter having totally different static and dynamic properties compared to the original converter.
A CF converter can be implemented by two different methods such as 1) adding an input capacitor at the input terminals of a VF converter and applying positive feedback from the input voltage as well as 2) by transforming a VF converter into a corresponding CF converter by applying duality transformation methods. The first method actually changes the properties of the converter in such a way that a buck-type VF converter becomes a boost-type CF converter and vice versa.
DC/DC converters used to interfacing purposes can be based either on voltage fed or current fed technologies. The current fed converters, the electrical dual of voltage fed converters, is still another , but less known and used, power circuit alternative for power converters over their voltage fed counterpart is that shoot through and half cycle symmetry cannot cause device failure or core saturation. This is characteristic of SCR based converters and one of the main reasons why current fed tends to be more robust.
The main disadvantage of current fed converter is that a fourth order conversion stage is required to convert DC bus voltage to a DC current. While the added stage results in additional complexity and losses, the power conversion stages can be made to work more efficiently. Current fed converters are implemented less than voltage fed converters primarily because of cost.
CF converter has less constraint than the VF converter in the MPP Tracing application. In reality many of the reported MPP tracking converters are CF converter.
The input voltage controlled CF converter can operate within the whole practical operation range of the solar generator. CF superbuck converter is interfacing with the solar generator to Extract Maximum power by using MPP Tracking Converters. In order to protect the storage battery or large capacitor bank connected at the output of a CF superbuck converter from damage.
PV Generator Characteristics
Solar energy can be utilized directly as heat power or by transforming it into electricity by means of solar or PV cells or solar chimneys [3]. The simplified electrical equivalent model of the solar cell composes of a photocurrent source and a diode connected in parallel with the current source as depicted in Fig. 1a. [5]. The corresponding theoretical simplified relation between the cell output current (iSC ) and voltage (uSC ) can be given as shown in (1) [5],[6]. This means that the maximum output voltage of a single-junction silicon cell is slightly less than the forward biased voltage of the parallel diode, which is typically in the order of 0.5 V - 0.7 V. As a consequence, several solar cells have to be connected usually in series as a solar panel (known also as module) and those modules further in series (known as a string) for enabling practical solar energy harvesting as depicted in Fig.1b, where ns is the number of series cells and np is the number of parallel cells (i.e., usually the number of parallel strings), respectively. The solar strings can be connected in parallel either directly or by means power-electronic-interfacing devices in order to increase the output power of the solar energy system. Such an entity is known as solar array and also generally as solar or PV generator.
Usc+rsisc
isc = iph-Is (e AVT -1) (1)
where I s is the diode dark saturation current, A diode ideality factor, V T silicon junction thermal voltage, r s current path series resistance, and rsh the shunt resistance, respectively. Basically, the output of the solar panel or string follows also the same relation as in (1) but modified with the number of series and parallel cells.
Fig. 1(a) Electrical equivalent models for a solar cell (b) static characteristics of U/I and P/U curves
According to Fig. 1, the PV generator is basically a current source up to the point at which the parallel connected diodes become forward biased as depicted in Fig. 2a [5],[6]. When a sufficiently large part of the photocurrent (i ph ) flows through the parallel diodes, the PV generator can be considered as a voltage source due to the behavior of the diode U/I curve as depicted in Fig. 2a (i.e., the dynamic resistance of the diode is quite small: See Fig. 2b) [7]. The point denoted by MPP in the I/U curve of Fig. 2a is known as the maximum power point according to the maximum power transfer theorem [8].
PV GENERATOR GRID INTERFACE
Usually there is a need for supplying the PV energy into a grid in an AC form. In the telecom and pace applications, the solar energy can be utilized also in a DC form due to the construction of the corresponding power systems. The output of the PV generator is DC in nature with the limitations discussed above. Therefore, power electronic converters have to be used for the interfacing tasks as discussed e.g. in [15]-[30]. Usually the interfacing is accomplished by connecting a DC/DC converter at the direct output of the solar generator and an inverter at the output of the DC/DC converter as depicted in Fig. 5a. Due to the desire to reduce the overall conversion losses, the inverters are sometimes connected also directly at the output of the PV generator (Fig. 5b) as shown in [25]. Even if Fig. 5 depicts the use of three-phase inverters, the inverter can be also a single-phase converter.
2a)
2b)
Fig. 2 Solar-generator-grid-interface principles: 2a) Double-stage conversion, and 2b) Single-stage conversion.
Solar energy is extensively utilized in the spacecraft applications [15]-[17]. The energy stored in the output capacitor of the PV generator (Fig. 1a) may cause excessive current spikes during the short-circuiting of the PV generator output. Those current spikes may increase EMI noise significantly and damage the active switch as discussed in [10]. It may be also clear that the output voltage of the regulator has to be much less than the open-circuit voltage of the solar generator for ensuring any regulation under the practical loading.